Protactinium tetraiodide

Protactinium tetraiodide is a dark green solid which can be prepared by reducing the pentaiodide with aluminum at about 400°C (44). The structure is not known. On sublimation in silica above 500°C small amounts of the pink oxydiiodide, PaOIg, are formed. Structural information is not available for the oxydiiodide, but it is known to be iso-morphous with ThOl2-... 

The iodide complexes are somewhat less stable, and well-characterized examples exist only for thorium, protactinium, and uranium. The thorium and uranium derivatives can be coveniently prepared by the reaction of iodine and metal, while protactinium tetraiodide is generated by reduction of Pals. The molecular structure of Thl4 has been examined the metal lies within a distorted square antiprism of iodide ions. 

Protactinium pentaiodide was first prepared by reacting the pentox-ide with aluminum triiodide at 400°C in a vacuum (104). For large scale preparations, however, direct union of the elements or metathesis of the pentaohloride or pentabromide with an excess of silicon tetraiodide are best (40). The latter reactions take place rapidly at about 180°C in vacuo and the black crystalline product is then purified by vacuum sublimation at 400°-450°C. Silicon tetraiodide also reacts with protactinium pent-oxide, but temperatures in excess of 600°C are required with the reaction vessel completely enclosed in the furnace, and the yield is only about 70% (40). 

Even fewer complexes with nitrogen donor ligands have been reported and all are methyl cyanide adducts (Tables X and XI). Protactinium pentabromide forms a soluble 1 3 complex in contrast to the 1 1 complexes formed by niobium and tantalum pentahalides (46). Other actinide pentahalide-methyl cyanide complexes are still unknown. Protactinium tetrachloride, tetrabromide, and tetraiodide react with anhydrous, oxygen-free methyl cyanide to form slightly soluble 1 4 complexes (44, 48) which are isostructural with their actinide tetrahalide analogs. 

Protactinium monocarbide was prepared by carbothermic reduction of the pent-oxide and treated with iodine (400 C), bromine (350 C), and thionyl chloride (200°C) in evacuated sealed tubes to give the appropriate pentahalide. The tetraiodide was obtained by interaction of the monocarbide with either protactinum pentaiodide (600 °C) or mercury(ii) iodide (500 °C) in vacuo. 

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